1. Field of the Invention
The present invention relates to an offshore vessel capable of installing and removing a subsea well control device and a riser string, the vessel comprising                a hull having a moonpool;        a mast having a top side and a base connected to the hull of the vessel, which mast is provided above or adjacent the moonpool;        a hoisting device supported by the mast for raising and lowering a load, such as a riser string, comprising a travelling block for connecting the load to the hoisting device, which travelling block is displaceable along a firing line which extends through the moonpool;        a heave compensation system associated with the hoisting device, for damping the effect of the movement of the vessel as a result of sea-state induced vessel motion on the load attached to the travelling block;        
a working deck supported by the hull of the vessel which covers at least a portion of the moonpool to allow the assembly of a riser string, and wherein the firing line extends through an opening in said working deck, such that the hoisting device can raise and lower a load, such as the riser string, through the opening in the working deck, wherein said working deck is provided with a riser string suspension device that allows to suspend a top end of a string of risers, preferably with a subsea well control device attached to the lower end of the riser string, from the working deck in the firing line.
2. Description of Background Art
Such offshore vessels comprising a heave compensated hoist system supported by a mast having a working deck provided over the moonpool are generally well known. The working deck is used to handle risers, which are connected and lowered by the hoist system to form a riser string. The heave compensated hoisting device is used to provide heave compensation for loads suspended from the travelling block.
The riser string, connected subsea well control device, and optionally connected well entry equipment form a stiff entity extending between the vessel and the bottom of the sea. Therefor, generally a riser tensioner is provided to manage the differential movements between the riser string and the vessel. If there were no riser tensioner and the vessel moves downward, the riser string would buckle; if the vessel rises then high forces would be transmitted to the riser string and it would stretch and be damaged. The riser tensioner may be a direct acting riser tensioner device, or to a riser tensioner ring of a cable type riser tensioner system is arranged on board of the vessel. As is known in the art these procedures may include the attachment of a slip joint or telescopic joint to the top end of the riser string.
Hence, it is common to provide a riser tensioner as a primary heave compensator between the vessel and the riser string, and use the heave compensation system associated with the hoisting device as auxiliary heave compensator when well entry equipment is to be attached to the riser string. For example, the injector head of a coiled tubing installation is connected to the heave compensated travelling block.
A disadvantage of such a vessel is that in order to operate the injector head, personnel needs to be brought at the level of the travelling block, generally using tuggers etc. This operation is disadvantageous.